In perturbative quantum field theory a *vacuum state* is the information needed to turn a product of field observables such as $\mathbf{\Phi}^a(x) \mathbf{\Phi}^b(y)$ into a function (or rather: generalized function/distribution) of the insertion points $x$ any $y$, namely the n-point function (here 2-point function, also called the *Hadamard propagator*)

$\langle \mathbf{\Phi}^a(x) \mathbf{\Phi}^b(y)\rangle$

which may be regarded as the probability amplitude for a quantum in state $b$ at spacetime point $y$ to turn into a quantum in state $a$ at spacetime point $x$, *in the given state* that the fields are in, which is defined thereby (see at *state in AQFT*).

In the worldline formalism of field theories these *propagators* arise from a 1-dimensional field theory on the “worldline” of (virtual) particles running from $y$ to $x$.

Now by the very definition of perturbative string theory, these particles are replaced by strings whose dynamics is now encoded in a 2d field theory on the worldsheet of strings, specifically a 2d superconformal field theory (2d SCFT) of central charge 15. Hence now it is the *2d SCFT* which defines the *vacuum state* that the perturbative string theory is in.

This is then called a *perturbative string theory vacuum*.

If this 2d SCFT arises from quantization of a sigma-model, then this is called a *geometric background*, otherwise it is a purely algebraically defined *non-geometric string vacuum*.

In practice full 2d SCFTs are hard to construct, and often one considers them by perturbation theory of a “sigma-model” which is defined by a spacetime manifold equipped with extra fields (e.g. the B-field etc.). It turns out that to low order these background field configurations that define sigma-model 2d SCFTs are given by solutions to equations of motion of supergravity theories (e.g. type II supergravity for type II string theory, etc.)

Therefore often such supergravity solutions equipped with some extra data that makes them consistent CFT backgrounds at higher order are referred to as *vacua* for string theory. But this is in general a coarse approximation. The full vacua are the full 2d SCFTs that define the worldsheet theory of the string.

The collection of all string vacua, possibly subject to some assumptions, has come to be called the *landscape of string theory vacua*.

See the references at

On (in-)stability of non-supersymmetric AdS vacua in string theory:

- Iosif Bena, Krzysztof Pilch, Nicholas Warner,
*Brane-Jet Instabilities*, J. High Energ. Phys. 2020, 91 (2020) (arXiv:2003.02851)

Last revised on January 2, 2021 at 09:22:09. See the history of this page for a list of all contributions to it.